The thermo-mechanical and metallurgical properties, microstructure, and etching characteristics of a plated copper film depend on the electroplating process used to produce the film. More specifically, these qualities depend upon bath additives, nature of agitation, filtration, current density, and the nature of the electrical contacts used to form the film. The microstructure of plated metals such as copper or gold is known to be unstable at room temperature. Room temperature grain recovery and grain growth processes are difficult to control, yet must be suppressed.
Uncontrolled grain recovery and grain growth may cause defects during subsequent processes. For example, the step of etching a plated metal film surface during the grain growth period may result in the etchant species selectively attacking the active grain boundaries. Thus, the grain boundary regions are etched faster than desired. The resulting rough metal surface may present difficulties in subsequent processes, such as forming another film on the metal film surface. A dielectric film may be deposited onto the metal film surface, for example, in a common device application. A rough metal surface may present difficulties in obtaining the desired qualities of the dielectric film being deposited over the etched copper. For example, when the film thickness of the dielectric film is being monitored as deposited using ellipsometric processes, the accuracy of the ellipsometric monitoring is reduced due to anomalous scattering from the highly decorated grains. This reduced accuracy is particularly troublesome when the dimensions of the grains of the plated film are comparable to those of the wavelength of the light used in the optical data collection.
It is also known that the impurity content of deposited metal films depends strongly on the concentration of additives in the plating formulation used in the plating bath. For example, to obtain a specific, desired amount of impurity such as carbon, oxygen, nitrogen, or sulfur, in a plated film, the impurity concentration in the bath formulation must be within a tight range. In order to increase or decrease the amount of impurities produced in the plated film, a different bath formulation normally is required: a bath formulation that contains a correspondingly increased or decreased amount of impurity additives.
What is needed is an improved process for producing an electrodeposited metal film with increased impurity content, such that the impurity content will be nearly independent of the concentration of bath additives. Thus, such a process is one object of the present invention. Another object of the present invention is a process which has suppressed grain growth characteristics. A related object is to provide a process yielding an electrodeposited metal film which has superior metallurgical, electrical, and thermo-mechanical properties.